Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 122034
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
234 PM CST Tue Dec 12 2017

.SHORT TERM...
234 PM CST

Through Wednesday...

The main forecast concern through the period will focus on the
next clipper system expected to move across the area on
Wednesday. This system may result in two short periods of snow
over portions of northern Illinois/northwestern Indiana on
Wednesday.

After a cold evening, warm air advection will begin to kick up
late tonight in association with our approaching clipper system.
As a result, temperatures will likely bottom out this evening and
then begin to raise a few degrees overnight. The fast speed of
this approaching clipper will also induce a rapid increase in
west-southwest winds a couple thousand feet off the surface by
early Wednesday morning. As these winds interact with the
baroclinic zone over the region, expect a decent band of
frontogenesis to set up by early Wednesday morning, especially
over eastern sections of Wisconsin and possibly as far south as
northeastern IL. This will likely drive a progressive band of
moderate to perhaps briefly heavy snow.

The main question that remains at this time is how extensive will
this band of snow be over northeastern IL? Model guidance does
not typically handle these events the best, and I think they may
be under doing the band of snow over northeastern Illinois early
Wednesday morning. As a result, I have continued to mention a
period of snow Wednesday morning, especially over northeastern IL
and portions of northwestern IN. Given the progressive nature of
this band of snow, I am not expecting large amounts of snow,
especially considering it may only last an hour or two in any
given area. However, given that a quick inch of snow could occur
with this band during the morning rush, it could result in rather
high impacts over the Chicago area.

Once this band of snow moves over the area early in the morning,
expect things to quite down for a period during the late morning
as the surface low begins to shift into northern IL. Temperatures
are likely to warm near, or even a couple of degrees above
freezing prior to the arrival of the low, but as it begins to
shift east of the area, expect the colder air to spill back
southward over the area during the afternoon as the winds become
gusty (35 to 40 MPH) out of the northwest. It also appears that
another decent band of snow will move quickly across the area with
the cold during the afternoon as the parent mid- level
disturbance shifts over the area. Similar to the morning activity,
it appears that this band of snow would also be short lived,
likely moving out of the area by mid to late afternoon.

Total snow amounts from both bands could amount to an inch or two,
especially over far northeastern IL. However, additional lake
effect snow showers may add to these totals some Wednesday night.

KJB

&&

.LONG TERM...
314 AM CST

Wednesday through Monday...

Overview...Another clipper system will bring mainly snow to the
region Wednesday. Accumulating snow is most likely in the Lake
Michigan collar counties.  Precipitation type could be an issue
along and south of I-80. Lake effect snow will impact southern Lake
Michigan Wednesday night into Thursday morning. While there are a
couple additional chances for precip through the weekend, I have low
confidence in seeing measurable precip. I have higher confidence in
a warming trend Friday into early next week.

Another clipper system will travel over the forecast area Wednesday,
and similar to the past few clippers, the placement of the low will
dictate precip type. Most models have settled on a path through
northern IL and northwest IN, while the NAM has a more northerly
path over southern WI and the lake.  I leaned toward the more
southern solutions. Areas north of the low should see light snow
Wednesday assuming we are able to saturate.  Like the system Monday,
the majority of snow may fall behind the low as a secondary upper
level vorticity streamer moves through.  If an f-gen band forms, we
could see another narrow band of moderate to heavy snow like we saw
Monday evening.

Areas along and south of I-80 may see mixed precip or periods of
rain as temperatures should climb into the mid to upper 30s.  If the
low is further north, mixed precip and warmer temperatures will also
shift north.

A very favorable lake effect snow regime sets up behind the low
Wednesday night. Delta T`s look to be around 17 degrees and flow
will be down the heart of the lake.  Unlike the last few lake effect
snow events, this one should impact northeast IL Wednesday evening.
Winds back to northwest pushing the band over Lake and Porter
Counties overnight. It`s a bit far out to pinpoint snow amounts, but
I`m thinking snow totals will probably be higher than currently
forecast wherever the band sets up. The band will then push east
Thursday morning.

An upper level trough passes overhead Friday, and it could force a
little snow. The GFS has much more QPF than other models so I capped
precip chances in the chance range. A surface ridge and warm air
advection follow the trough leading to high temps in the upper 30s
to low 40s Saturday. Another low moves over the upper Great Lakes
over the weekend, but it looks like the majority of precip will fall
north of the forecast area.

JEE

&&

.AVIATION...
For the 18Z TAFs...

Northwest winds over the area this afternoon will continue to ease
through the afternoon, but the main focus during the period is
with another clipper system set to shift over the area on
Wednesday. The current track of this low would put if over
northern IL by midday Wednesday. Given this track, the wind
forecast will be tricky at the terminals as any minor fluctuation
in the expected track of this low could result in changes in the
favored wind directions and speeds. Currently we expect the winds
to become south-southeasterly by early Wednesday morning, then
shift west-southwesterly later in the morning as the low
approaches, and finally back to the northwest again during the
afternoon.

In addition to the winds, it appears that they may be a period or
two of snow over the eastern terminals on Wednesday. While the
better chances for snow is likely to remain north of the area in
Wisconsin, it does appear that there could be a short 1 to 3 hour
period of snow early Wednesday morning over northeastern IL in
association with increasing warm air advection. There is still
uncertainties in the extent of this band of snow, but if it
materializes could produce a short period of low visibilities in
moderate to heavier snow before it quickly shifts out of the area
by mid to late morning. While the snow is likely to end during the
morning, it appears that another quick moving band of snow could
shift over the terminals during the afternoon, with another period
of reduced visibilities possible. Given uncertainties in timing
and the extent of the snow, I have only mentioned the lowest
visibilities in tempos at this time.

KJB

&&

.MARINE...
314 AM CST

Current gale headlines look on track. Small Craft Advisories will be
needed for the nearshore waters after the gale warnings as winds and
waves slowly subside.

The low over the Eastern Great Lakes will continue to New England
today allowing a high pressure ridge to pass overhead tonight. Gales
will diminish late this afternoon into the early evening as the
pressure gradient relaxes between the departing low and approaching
ridge.

The next low pressure system will pass just over or just south of
the southern tip of Lake Michigan Wednesday afternoon. Gales are not
expected at this time, but gusts up to 25 kt are expected over the
southern half of the lake mid Wednesday morning through Wednesday
night. West winds are then forecast through Friday. Another weak low
is forecast to develop over Lake Superior Thursday evening and then
deepen as it moves over the Eastern Great lakes Friday evening.
Winds may increase to 30 kt over the southern end of the lake.  I
have low confidence in the forecast Saturday and beyond due to
significant differences in how guidance handles the pattern
progression.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Lakeshore Flood Advisory...INZ002 until 3 PM Tuesday.

LM...Small Craft Advisory...IL nearshore waters until 10 PM Tuesday.

     Gale Warning...IN nearshore waters until 3 PM Tuesday.

     Gale Warning...LMZ777-LMZ779 until 8 PM Tuesday.

&&

$$

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